The invention relates to automatic sprinklers and more particularly to automatic sprinklers having an on-off function.
Automatic on-off sprinklers, examples of which can be found in U.S. Pat. Nos. Re. 29,155; 3,757,866; 4,553,602; 4,368,782 and 3,848,676, activate automatically in response to a sensed temperature threshold and de-activate when the temperature lowers to a second threshold, e.g as the fire is suppressed. A typical automatic on-off sprinkler, e.g. as shown in FIG. 1, has a fire retardant fluid inlet passage 13, an outlet passage 14 and a passage 32. When an increase in the surrounding temperature causes bimetal disc 126 to reach an "on" threshold, the bimetal disc responds by snapping over to a reverse shape and opening the flow path from chamber 40 to passage 32. Flow of fluid through passage 32 lowers the pressure in chamber 40 and the corresponding side of piston 42, and the piston, in response to the differential pressure acting at opposite sides, moves inward to chamber 40 to allow flow between inlet passage 13 and outlet passage 14.
When the surrounding temperature drops, causing the temperature of the bimetal disc 126 to be lowered to its "off" threshold, the bimetal disc snaps back to its original shape, thereby closing passage 32. As a result, pressure in chamber 40 and on the corresponding side of piston 42 increases to return the piston to its original position, shutting off the flow between inlet passage 13 and outlet passage 14.
If the fire should re-establish itself, a subsequent increase in the surrounding temperature sufficient to raise the temperature of bimetal disc 126 to its "on" threshold causes the bimetal disc to again snap over to a reverse shape and open the flow path from chamber 40 to passage 32. As described above, flow of fluid through passage 32 lowers the pressure on the corresponding side of piston 42, and the piston, in response to differential pressure acting at opposite sides, moves to allow flow of fire extinguishing fluid between inlet 13 and outlet 14.